Coaxial non-rotating linear reciprocating members in power conversion machinery, such as Stirling cycle machines, incorporate coaxial reciprocating elements with associated internal and/or external seals. The sealing functions are typically provided by means of sliding or rubbing surfaces in contact with one another, which result in wear, detrimental seal leakage and machinery lifetimes of uncertain duration.
Means previously identified for avoiding these life and reliability limitations include 1) gas bearing supports/seals, 2) lubricated bearings with hermetic bellows seals to prevent lubricant ingress to the working cycle region, and 3) flexural bearings used in conjunction with clearance seals.
The present invention arose from an effort to improve the implementation of flexural bearings and clearance seals. The general advantages of flexural bearings relative to gas bearings include the following: lower cost resulting from reduced precision manufacturing steps; higher reliability resulting from elimination of ports subject to plugging and reduction of sensitivity to very small particles; less frictional wear and less generation of unwanted debris resulting from elimination of rubbing contact during startup and shutdown; provision of some or all of the axial spring force required to resonate the moving component; and reduced complexity by avoiding the gas bearing actuation function and in some cases eliminating a gas return spring.
The existing state of the art in flexural bearings and clearance seals is well illustrated by U.S. Pat. No. 4,475,335. It illustrates use of stacks of circular sheet metal flexures with three legged spiral kerfs between an outside diameter clamp ring and an inside diameter clamp ring, such that the flexures function as bearing supports.
Two flexure bearing stacks are axially displaced one from another in the referenced patent disclosure. Both are clamped rigidly near their outside diameter in a common housing. The inner diameters are similarly affixed to a reciprocating rod which is relatively free to move axially. The flexure bearings rigidly resist any tendency toward radial motion.
A reciprocating linear drive motor is disposed between or outboard of the flexure bearing stacks and affixed to the rod such that it can impart forced oscillation of the rod, typically at a frequency which is resonant with the mass-spring-damper system natural frequency of the reciprocating subassembly.
A piston is attached to a cantilevered extension of the rod axially beyond the set of flexure bearings. The piston reciprocates within a surrounding cylinder which is rigidly coupled to the bearing housing and constrained to be substantially coaxial with the flexure bearing supports. A very tight clearance seal between the piston and cylinder is provided to minimize cyclic leakage of the working gas between the regions at each end of the piston.